The present invention relates to an image fixing device for fixing an image on an image carrying member, and a method for fixing the image, and more particularly, to the image fixing device and method for forming a transparent protective coating layer over an image formed on the image carrying member.
In a conventional electrophotographic method, an electrostatic latent image is formed upon image exposure on a photosensitive member whose surface has been uniformly charged with electricity by a corona discharge, and then wet type developing is conducted by using a liquid developing agent. In the liquid developing agent, only a small amount of image fixing resin is contained, and therefore, the resultant fixed image is easily damaged and contaminated with foreign objects. Further, if the photosensitive member is formed of a photosensitive material in which photoconductive minute particles such as zinc oxide and titanium oxide particles are dispersed in a resin binder, a surface of the photosensitive member is of porous structure. Therefore, an output image formed on the surface of the porous photosensitive member is not flat but becomes matte form. Accordingly, diffused reflection may occur at the surface of the output image, and consequently, high density output image may not be obtainable.
To overcome this drawback, there is known a method for forming a protective coating layer formed of a resin binder over the output electrophotographic image for image fixing. For example, according to one conventional image fixing method, a resin solution in which a resin binder is dissolved in an organic solvent is coated on a surface of the output image, and then the solution is subjected to drying to obtain a transparent protective layer. However, in this method, a conflicting problem may occur in view of the transparency and mechanical strength of the protective coating layer on one hand and insolubility of the photosensitive material and image fixing resin into the solvent of the resin solution which constitute the protective coating layer on the other hand. That is, the protective coating layer must provide sufficient strength and transparency. On the other hand, the output image must not be degraded due to the solubility of the photosensitive material and the image fixing resin into the solvent which is used for dissolving the resin binder for forming the protective layer. Taking the two aspects into consideration, the available kind of solvent undergoes restriction so as to meet with the conflicting requirement of transparency and insolubility.
Further, if the electrophotographic output image is obtained by the wet type developing method, and if the above described method is conducted for forming, the protective coating layer over the image, another disadvantage may occur in that two solvent drying processes are required, which leads to excessive energy consumption, and further, the solvent is evaporated in the atmosphere due to the drying process which may cause environmental pollution.
Another conventional method for forming the protective coating layer is known. In this method, thermal melting type transparent resin layer is formed over a substrate sheet, and the substrate sheet carrying the resin layer is thermally melted on the output image surface and thereafter, the substrate sheet is solely peeled off for finally forming a transparent resin layer over the surface of the output image. However, this method requires a separate substrate sheet whose area must be the same as that of the electrophotographic output image. Therefore, a great number of substrate sheets must be stored in an image forming apparatus. Further, the resultant output image becomes costly, since the substrate sheet is finally removed.